PETERBOROUGH — Since mid-April, construction trailers have lined the former railroad bed at the edge of the New Hampshire Ball Bearings plant on Route 202, as workers install tubing to create what’s known as a “permeable reactive barrier” — an underground wall of iron particles intended to treat contaminated groundwater.

“It’s like the fence at the edge of a ballpark,” said Chris Rawnsley, the manager of environmental affairs for New Hampshire Ball Bearings, during a visit to the site last week. “We’ll also be doing a thermal radiation process to treat the soil and groundwater near the building and bioremediation, which treats the soil and water with bacteria.”

Rawnsley is monitoring the work being done by GeoSierra Environmental Inc., a Medford, N.J., company that is building the barrier.

The three-step treatment process is the latest stage of the cleanup of the South Municipal Water Supply Well Superfund Site, which has been ongoing since 1982, when contaminants were found in the water during testing at Peterborough’s South Well. The well is located on the opposite side of the Contoocook River from the NHBB building. Chlorinated solvents used in the ball bearings manufacturing process at the plant, which was built after NHBB acquired the 24-acre site in 1955, were identified as the source of the contamination. The well was closed and NHBB conducted a remedial investigation/feasibility study under the supervision of the Environmental Protection Agency and the N.H. Department of Environmental Services. In 1990, the EPA issued an order directing NHBB to clean up the site at the company’s expense.

The early remediation effort consisted of pumping water from wells dug on the NHBB property, then treating it and releasing it in wetlands on the property. But tests conducted by the town at the South Well in 2005 showed that method was ineffective and in 2010 the EPA modified its decision, requiring NHBB to use thermal remediation, bioremediation, and the permeable reactive barrier.

The estimated cost of the project is about $13.6 million, which will be entirely paid by NHBB.

“This is new technology,” Rawnsley said. “Our intent is to allow use of the South Well. That’s what we’ve been mandated to do.”

Rawnsley said it is difficult to predict how long the new remediation process will take. The permeable reactive barrier that’s currently under construction should be complete by late August or early September, but work has not yet started on the thermal remediation and bioremediation steps.

The permeable reactive barrier is being built using a newly developed process that doesn’t require digging a trench. Instead, GeoSierra Environmental is installing a series of tubes along the former railbed, which is downhill from the NHBB plant. Each tube goes about 50 feet down through the earth until bedrock is hit. Then a fluid called guar, which contains iron particles, is injected down the tubes and will spread between them. The guar will disintegrate, leaving behind a 3-inch thick, 350-foot long wall of installed iron that can serve as a filter for groundwater that drains down the slope.

“It’s basically sand made out of iron that we’re putting in,” Rawnsley said. “It’s remarkably simple, but also remarkably expensive.”

He said the technology, which has been developed in the last 10 to 15 years, wasn’t an option when NHBB first started working with the EPA on plans for cleaning the site.

Kevin Heine, the Boston-based remedial project manager for the EPA who is overseeing the project, said the remediation will take time to complete.

“The thermal remediation will start in 2015,” Heine said. “The bioremediation is intended to stimulate microbes that exist in the soil that could deal with residual contaminants. Then you need some time for water to flow through and be treated.”

He said both the EPA and DES will have to analyze data after the three remediation steps have been completed.

“We’re talking many years,” Heine said. “There’s no fixed date to get it all done.”

Peterborough Public Works Director Rodney Bartlett said last week that the town’s three other wells will satisfy the town’s current demand, now that voters have approved building a new well and pump house on Summer Street to replace the existing well, which was no longer drawing adequately.

“But if one of our wells goes down, then supply is marginal,” Bartlett said. “Having that fourth source at the South Well is our long-term goal.”

Dave Anderson can be reached at 924-7172, ext. 233 or danderson@ledgertranscript.com. He’s on Twitter at @DaveAndersonMLT.